The disclosure of Japanese Patent Application No. 2000-311244 filed on Oct. 11, 2000, including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to a vehicular shift control unit and vehicular automatic shift control method which controls a cooperative shift of an automatic transmission based on an information associated with a condition around a vehicle or a condition in front of the vehicle.
2. Description of Related Art
There has been well known a normal automatic shift control for controlling a gear stage of an automatic transmission so as to satisfy a gear stage determined based on a vehicle speed and an accelerator opening degree in accordance with a shift diagram. Further, there has been known a vehicular shift control unit for executing a cooperative shift control for controlling the automatic transmission to a different gear stage from the gear stage executed in accordance with the automatic shift control based on the information indicating the condition around the vehicle or in front of the vehicle, in addition to the automatic shift control. The information indicating the condition around the vehicle or in front of the vehicle is obtained by a navigation apparatus, a transmitting apparatus or the like provided in a road. According to this cooperative shift control, a gear ratio or a gear stage is controlled so as to obtain a driving force suitable for a road shape around the vehicle or in front of the vehicle. The vehicular shift control unit is, for example, constituted by a cooperative shift control unit described in Japanese Patent Application Laid-open No. 2000-142180. According to this structure, a radius of curvature of a curve is sequentially calculated so that an engine brake force capable of obtaining an optimum vehicle speed corresponding to a traveling direction of the vehicle, that is, the curve of the road ahead can be obtained, so as to determine a recommended gear ratio corresponding to the radius of curvature R. Accordingly, the vehicle can safely travels on a curved road. Further, the cooperative shift control unit calculates a speed reduction curve based on the recommended gear ratio, and compares an actual vehicle speed with the speed reduction curve. Next, the cooperative shift apparatus determines an upshift inhibition or a downshift based on the comparison, and executes the determined shift of the automatic transmission after releasing an accelerator pedal or operating a brake before entering a curve area.
In the vehicular shift control unit as mentioned above, the traveling path of the vehicle, that is, a destination road is definite during a period in which the traveling route in front of the vehicle is guided using the information from the navigation system. Accordingly, the gear ratio corresponding to the road curve can be obtained. However, since the destination path of the vehicle is not definite when the vehicle is not under route guidance, it is impossible to determine which branch road the vehicle should take when the vehicle approaches a branch point of the road. Accordingly, it is impossible to obtain road information data of the branch point transmitted from the navigation system and there is a possibility that the cooperative shift control is cancelled in the middle thereof. Therefore, a low speed gear stage is not continuously maintained in accordance with the cooperative shift control during traveling on the curved road, and there is a possibility that an upshift to a high speed stage is accidentally executed.
An object of the invention is to provide a vehicular shift control unit and a method for controlling the same in which a shift state executed by a cooperative shift control is continuously maintained during traveling on a curved road even when not being guided along a path.
In accordance with a first aspect of the invention, there is provided a vehicular automatic shift control unit that executes a cooperative shift control of an automatic transmission based on an information associated with at least one of a road condition around a vehicle and a road condition in front of the vehicle, including a judging apparatus that judges whether the vehicle exists in a predetermined area including a branch point of the road during execution of the cooperative shift control; and a gear ratio maintaining controller that maintains a gear ratio of the automatic transmission if it is judged that the vehicle exists in the predetermined area.
In accordance with the first aspect, the current gear ratio of the automatic transmission is maintained during a period that a vehicle travels within the predetermined area including the branch point. Accordingly, the vehicle can travel near the branch point in a state of maintaining the gear ratio of the automatic transmission applied during the cooperative shift control. Therefore, the shift state applied by the cooperative shift control during traveling on the curved road is continuously maintained. Accordingly, a drivability during traveling along a corner is not deteriorated.
In accordance with a second aspect of the invention, there is provided a vehicular automatic shift control method for executing a cooperative shift control of an automatic transmission based on an information associated with at least one of a road condition around a vehicle and a road condition in front of the vehicle, including a step of judging whether the vehicle exists in a predetermined area including a branch point during execution of the cooperative shift control; and a step of maintaining a gear ratio of the automatic transmission if it is judged that the vehicle exists in the predetermined area.
In accordance with the second aspect, in the same manner as the first aspect, the vehicle can travel near the branch point in a state of maintaining the gear ratio of the automatic transmission applied during the cooperative shift control. Accordingly, a drivability during traveling along a corner is not deteriorated.